As is known in the art, prior to the invent of global positioning satellite (GPS) technology, accurate timing information was distributed by transmitting stable reference clock signals between different transmitter and receiver sites with each site having a stable reference clock free-running between synchronizations. Portable reference clocks were moved between locations to provide the necessary periodic synchronization. An alternative approach was to use time broadcasts from reference stations. The National Institute of Standards and Technology (NIST), for example, broadcasts reference clock signals as do time reference stations by other nations. Unfortunately, however, timing information distributed by NIST through transmissions of reference clock signals at WWV in Ft. Collins, Colo. and WWVH in Kauai, Hi., for example, as well as timing information distributed by other time reference stations, was never quite accurate enough to be used in precise timing applications for reasons that will shortly be discussed.
Given heavy reliance on GPS technology and the timing information distributed from such technology, there is present concern over how accurate timing information can and should be generated and distributed in the event that something should happen to GPS. This is especially true since the infrastructure for distributing timing information by free-running stable clocks and portable reference clock signals is largely no longer in place. For many time sensitive applications, high speed internet and accurate Network Time Protocol are adequate for distributing timing information. For other applications, however, such are inadequate. Consequently, there is a desire to increase the accuracy of received timing information that is distributed by the NIST radio stations (hereinafter, “NIST”), which broadcasts timing and frequency information traceable to an atomic time standard, as well as timing information distributed by other time reference stations.
NIST, for example, transmits high-frequency (HF) timing and frequency information 24 hours per day, 7 days per week from WWV and WWVH on 2.5, 5, 10, and 15 MHz frequencies, with each frequency being broadcast from a separate transmitter. WWV is also capable of transmitting timing and frequency information at a frequency of 20 MHz, while both WWV and WWVH are capable of broadcasting marine warnings, GPS status reports, leap second information, and geophysical alerts. The quality of HF signal reception in general varies from transmission to transmission due to a wide variety of factors including, but not limited to, atmospheric and ionospheric propagation conditions, distances from the transmitters to the receivers, and time of day, with different frequencies more easily received at different times of the day. Additionally, there are certain well-known difficulties which make HF radio transmission unreliable when only a single frequency is transmitted. For these reasons each frequency transmitted from NIST carries substantially the same information as the other frequencies transmitted, with multiple frequencies being used to improve HF signal reception. Utilizing a variety of frequencies makes it likely that a signal from at least one of the frequencies will be usable at all time.
One limitation on distributing accurate timing information from NIST, in addition to the above described issues with the quality of HF signal reception, is largely the uncertainty of the time delay from a transmitter at NIST to a receiver at a second location with the HF signal being “bounced” off of the earth's ionosphere. The received signal time must be compensated for the time required for the signal to propagate from NIST to the receiver. Complexities can also arise due to the existence of many potential propagation modes from the transmitter at NIST to the receiver. Distributing accurate timing information, and forming an absolute time reference, accordingly requires understanding the propagation mode over which the signal is received and compensating for such delay. Furthermore, the propagation mode and delays associated with each transmission from NIST changes over time. As a result, a new compensation value must be applied to each received transmission to match the particular environment and transmission path over which a signal with the timing information is sent.
Therefore, what is needed is a more robust receiver system for receiving and processing the accurate timing information transmitted by and received from NIST and the like, taking into account the issues discussed above.